Commonly, construction machine, such as a hydraulic excavator and the like, includes an engine as a prime mover, a hydraulic pump driven by the engine, and hydraulic actuators for a boom cylinder, bucket cylinder and/or the like which are operated by pressure oil discharged from the hydraulic pump. The hydraulic actuator is operated to actuate a front working device such as a boom, a bucket or the like mounted in the front of the vehicle body in order to perform required work such as excavation, dumping or the like.
In the construction machine having such a configuration, mud and the like adhering to a crawler are removed by stranding the machine on a bump on a road surface in the travel direction during the working or alternatively by causing the crawler of the undercarriage to be idle. To that purpose, the jack-up operation is performed to jack up the vehicle body by pressing the bucket against the ground with the boom lowering motion. Then, there is a need in the conventional art for hydraulic equipment that is capable of causing the boom to produce a great pressing force without loss of intended operability of the boom lowering motion.
Known as a conventional technique including hydraulic equipment of this type is a hydraulic drive system for construction machine which has: a control valve including an open-center type directional control valve to control a flow of pressure oil to be supplied from a hydraulic pump to a hydraulic actuator; and an operating device to operate switching of the directional control valve, in which the control valve has two directional control valves with different operational performances for each section of the hydraulic actuator and the hydraulic drive system is equipped with signal switching means for selecting one of the two directional control valves to which an operating signal from the operating device is directed (see, e.g., Patent Literature 1).
Also known as another conventional technique is a hydraulic circuit for a hydraulic working mechanism which includes: a directional control valve to control a flow of pressure oil flowing to a boom cylinder; and an operating device to perform switching operation for the directional control valve, as well as: a jack-up selector valve which is switched when a bottom pressure of a boom cylinder reaches a predetermined pressure; flow-path changing means for changing, to an open path or a closed path, a flow path of pressure oil to be supplied to a meter-in section of the directional control valve in step with the switching operation for the jack-up selector valve; and a slow return circuit including a throttle and a check valve to control the flow path of pressure oil to switch the jack-up selector valve (see e.g., Patent Literature 2).